Photoluminescent semiconductor nanocrystal-based luminescent solar concentrators

ABSTRACT

The present disclosure describes luminescent solar concentrators that include photoluminescent nanoparticles. The photoluminescent nanoparticles include a semiconductor nanocrystal that sensitizes the luminescence of a defect. The defect can include, for example, an atom, a cluster of atoms, or a lattice vacancy. The defect can be incorporated into the semiconductor nanocrystal, adsorbed onto, or otherwise associated with the surface of the semiconductor nanocrystal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Patent Application No.61/841,887, filed Jul. 1, 2013, and U.S. Patent Application No.61/895,224, filed Oct. 24, 2013, the disclosures which are herebyincorporated by reference in their entireties.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under DMR-1035512 andDMR-1206221, both awarded by the National Science Foundation. TheGovernment has certain rights in the invention.

BACKGROUND

Luminescent solar concentrators (LSCs) collect and concentrate sunlightfor use in solar power generation. LSCs are devices typically consistingof a planar waveguide coated or impregnated with a luminophore. Sunlightabsorbed by the luminophore coated on or contained within the waveguideis re-emitted into the waveguide, where it is captured by total internalreflection, which causes it to travel to the edges to be concentratedfor use by light conversion devices, such as photovoltaic cells (PVs).Unlike lens- and mirror-based concentrators, which require trackingsystems to follow the sun's motion and can only concentrate direct,specular sunlight, LSCs are passive devices that work equally well withboth diffuse and specular sunlight. They are therefore less costly tobuild, install, and maintain, more easily integrated into the builtenvironment or portable solar energy systems, more damage tolerant, andcan be used in climates where there is little direct sunlight.Furthermore, because LSCs can produce wavelength-to-bandgap matchedphotons by downshifting, there is reduced need for PV cooling, andmultiple LSC waveguides each incorporating a different luminophore canbe stacked to split the solar spectrum for tandem multi-cell conversion.

An exemplary LSC 10 is illustrated in FIG. 1, wherein the planarwaveguide 12 comprises a plurality of luminophores 15. The planarwaveguide 12 is edge-coupled to PV cells 14 sensitive to the emissionwavelength of luminophores 15. As shown in the detail of FIG. 1,luminophores 15 absorb light 16 of a first wavelength, and emit light 18of a second, red-shifted, wavelength. The emitted light 18 is used togenerate electrical current in the PV cells 14.

In combination with bandgap-matched, high-efficiency PV cells, LSCsoffer the potential for a reduction in the cost of solar electricity—bywell over an order of magnitude. However, LSCs have thus far had littlepractical impact, primarily because the optical quantum efficiency (OQE)decreases rapidly with concentrator size (i.e., as an LSC increases insize, a smaller fraction of incident sunlight is concentrated at theedges). Several factors contribute to a decreasing OQE, two of which areusually dominant: (i) photon loss due to non-unity photoluminescencequantum yield (QY) of the luminophore and (ii) loss of photons from atop and bottom of the waveguide from emission at an angle inside thecritical escape cone of the waveguide material, as defined by Snell'sLaw. For example, a typical organic luminophore-based LSC and apoly(methylmethacrylate) or glass waveguide can have a loss rate due tofactor (i) that is near zero, but a loss rate due to factor (ii) that isabout 25% per-emission. A decrease in OQE can also occur due tore-absorption and re-emission, as a captured photon traveling toward awaveguide edge may encounter other luminophores to be re-absorbed andre-emitted multiple times, and a fraction of photons can be lost witheach successive re-absorption/re-emission event due to non-radiativerelaxation processes. Repeating escape cone and QY losses thus compoundwith distance.

Previous efforts to address this problem have included the use oflarge-Stokes-shift luminophores that have a large energy differencebetween absorption and emission to reduce light re-absorption. Examplesof such luminophores include certain organic dyes, quantum dots,lanthanide and transition metal-based molecules, and microcrystallinephosphors, and organic dyes whose emission is red-shifted by solid-statesolvation. However, these luminophores tend to have low QY, narrow orweak spectral absorption bands that capture only a small portion of thesolar spectrum, limited environmental lifetime, large scattering crosssections, or a combination of these shortcomings. Other methods forimproving efficiency include the use of wavelength-selective mirrors andoriented luminophores for directing a larger portion of luminescenceinto waveguide modes and out of the escape cone. However, none of theseapproaches has proven successful in producing large area, highefficiency LSCs with long environmental lifetimes.

Accordingly, a large area, high efficiency LSC with a long environmentallifetime is needed. The present disclosure seeks to fulfill this needand provides further related advantages.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In one aspect, this disclosure features an LSC including a plurality ofphotoluminescent nanoparticles. Each photoluminescent nanoparticleincludes a semiconductor nanocrystal and a nanocrystal defect. Thenanocrystal defect and the semiconductor nanocrystal combine to producea photoluminescence effect. The defect can include an atom, a cluster ofatoms, a lattice vacancy, and any combination thereof. The waveguidematerial has the plurality of photoluminescent particles suspendedtherein or applied to a surface thereof.

In another aspect, this disclosure features a window pane, a coating, afree-standing polymer film, an electronic display, and/or a touch screenincluding the LSC.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of a representative luminescentsolar concentrator (LSC) as known in the prior art.

FIG. 2 is a schematic representation of an embodiment of an LSC.

FIG. 3A is a schematic representation of an emission process of anembodiment of an LSC.

FIG. 3B shows two photoluminescence (PL) decay curves of an embodimentof photoluminescent nanoparticles. The two decay curves areindistinguishable, consistent with negligible thermally activatednonradiative decay. The photoluminescent nanoparticles have a 100%photoluminescence quantum yield (QY), measured at 290 and 77 K.

FIGS. 4A-4C are schematic representations of embodiments ofphotoluminescent nanoparticles. FIG. 4A is a schematic representation ofa monolithic photoluminescent nanoparticle. FIG. 4B is a schematicrepresentation of a photoluminescent nanoparticle having a coresurrounded by three shell layers. FIG. 4C is a schematic representationof a photoluminescent nanoparticle having a core surrounded by a shell.

FIG. 5 shows absorption spectra of embodiments of photoluminescentnanoparticles. The photoluminescence maximum at 2.1 eV corresponds to˜590 nm.

FIG. 5 also shows a photoluminescence excitation (PLE) of an embodimentof photoluminescent nanoparticles.

FIG. 6 is a scanning electron micrograph of an embodiment ofphotoluminescent nanocrystals.

FIG. 7 is an energy diagram of embodiments of defect and semiconductormaterial combinations for photoluminescent nanoparticles.

FIG. 8 is a graph showing the effect of waveguide losses on the OQE of a1 m² LSC and the attenuation coefficients for waveguide materials.

FIG. 9 is a schematic illustration of an embodiment of a multilayeredLSC.

FIG. 10 is a schematic illustration of an embodiment of a multilayeredLSC.

FIG. 11 is a schematic representation of an experimental setup formeasuring LSC OQE as a function of geometric gain.

FIGS. 12A-12C are normalized absorption (ABS) and emission (EM) spectraof polymer film LSCs including embodiments of photoluminescentnanoparticles. FIG. 12A: LSC including Mn:ZnSe/ZnS photoluminescentnanoparticles. FIG. 12B: LSC including Mn:CdZnSe/ZnS photoluminescentnanoparticles. FIG. 12C: LSC including Cu:CdSe/CdS photoluminescentnanoparticles.

FIG. 13A-13C are graphs showing OQE variation with geometric gain forembodiments of photoluminescent nanoparticles. FIG. 13A: Mn:ZnSe/ZnSphotoluminescent nanoparticles. FIG. 13B: Mn:CdZnSe/CdS photoluminescentnanoparticles. FIG. 13C: Cu:CdSe/CdS photoluminescent nanoparticles.

FIGS. 14A-14C are emission spectra collected through the edge apertureas a function of geometric gain for polymer LSCs including embodimentsof photoluminescent nanoparticles. Emission spectra were collected usingthe experimental arrangement illustrated in FIG. 11. Each linerepresents a different position of the mask, ranging from 5 to 75 mm asmeasured from the left-hand side of the mask to the collection edge. Thecorresponding geometric gain ranged from 7.8 to 20. FIG. 14A: LSCincluding MnCdZnSe/CdS photoluminescent nanoparticles. FIG. 14B: LSCincluding Mn:ZnSe/ZnS photoluminescent nanoparticles. FIG. 14C: LSCincluding Cu:CdSe/CdS photoluminescent nanoparticles.

FIGS. 15A and 15B are normalized absorbance (ABS) and emission (EM)spectra of liquid-filled LSCs including embodiments of photoluminescentnanoparticles. FIG. 15A: LSC including Mn-doped core-shell ZnSe/ZnSphotoluminescent nanoparticles. FIG. 15B: LSC including alloyZn_(1-x-y)Cd_(x)Mn_(y)Se photoluminescent nanoparticles.

FIGS. 16A and 16B are normalized emission spectra taken as a function ofexcitation distance from the edge of liquid-filled LSCs includingembodiments of photoluminescent nanoparticles. LSCs were illuminatedusing a point excitation source. Each line represents an emissionspectrum collected using a different distance between the excitationpoint and collection edge, with the distance ranging from 0.01 in. to2.0 in. in FIG. 16A, and from 0.3 in. to 1.3 in. in FIG. 16B. FIG. 16A:LSC including Zn_(1-x-y)Cd_(x)Mn_(y)Se photoluminescent nanoparticles.FIG. 16B: LSC including Mn-doped core-shell ZnSe/ZnS photoluminescentnanoparticles. The spectra exhibit no significant red-shifting withincreasing excitation distance, indicating little to no self-absorption.

FIG. 17 is a graph showing normalized OQE as a function of excitationdistance from the device edge for two photoluminescentnanoparticle-based LSCs.

FIG. 18 is a graph comparing normalized waveguided photoluminescenceintensities as a function of the distance between excitation andcollection for an embodiment of photoluminescent nanoparticles of thepresent disclosure and an embodiment of photoluminescent nanoparticle ofprior art. The data show considerably smaller waveguide losses for theembodiment of photoluminescent nanoparticles of the present disclosure.

DETAILED DESCRIPTION

The present disclosure describes LSCs that include photoluminescentnanoparticles. The photoluminescent nanoparticles include asemiconductor nanocrystal that is associated with a defect. The defectcan include, for example, an atom, a cluster of atoms, or a latticevacancy. When the defect is an atom or a cluster of atoms, the defectcan be incorporated into the semiconductor nanocrystal, adsorbed onto,or otherwise associated to the surface of the semiconductor nanocrystal.When the defect is a lattice vacancy, the defect can be present withinor on a surface of the semiconductor nano crystal.

The semiconductor nanocrystal and the defect combine to provide aphotoluminescent material. For example, the semiconductor nanocrystalcan absorb photons and transfer its energy to the defect, which can thenemit light at a wavelength that is different from the absorptionwavelengths. In an idealized scenario, the photoluminescentnanoparticles should maximize absorption of incident irradiation (e.g.,solar irradiation) and emission, should have a photoluminescence QY thatapproaches as close to 100% as possible, and should have minimalre-absorption and scattering of emitted light.

It is believed that one of the benefits of the present photoluminescentnanoparticles is the wide spectral gap between the wavelengths at whichthe photoluminescent nanoparticles absorb and emit light, which leads tolittle to no self-absorption. Another benefit of the photoluminescentnanoparticles is their rapid energy localization at the defect, whichcan outcompete other trapping, photochemical, or nonradiative decayprocesses that reduce the photoluminescence QY or degrade the materials.Other benefits of the photoluminescent nanoparticles include superiorenvironmental lifetime compared to organic luminophores as a result oftheir resistance to photochemical bleaching; the use of abundant,low-cost starting materials in their preparation; their compatibilitywith a wide range of solvents, waveguide matrix materials, andprocessing methods; reduced light scattering compared to luminophoresthat include certain other nanoparticles, and in many cases the absenceof toxic elements or other potentially harmful compounds. Thephotoluminescent nanoparticles can absorb light over a broad spectralrange. In addition, the absorbance range and emission spectrum can eachbe tuned through the composition, structure, or size of thenanoparticles.

Definitions

At various places in the present specification, substituents ofcompounds of the disclosure are disclosed in groups or in ranges. It isspecifically intended that the disclosure include each and everyindividual subcombination of the members of such groups and ranges. Forexample, the term “C₁₋₆ alkyl” is specifically intended to individuallydisclose methyl, ethyl, C₃ alkyl, C₄ alkyl, C₅ alkyl, and C₆ alkyl.

It is further appreciated that certain features of the disclosure, whichare, for clarity, described in the context of separate embodiments, canalso be provided in combination in a single embodiment.

Conversely, various features of the disclosure which are, for brevity,described in the context of a single embodiment, can also be providedseparately or in any suitable subcombination.

As used herein, “photoluminescent” refers to light emission from amaterial after the absorption of photons and encompasses fluorescenceand phosphorescence.

As used herein, “nanocrystal” refers to a crystal having its largestdimension smaller than or equal to about 100 nm, and composed of atomsin a crystalline arrangement.

As used herein, “nanoparticle” refers to a particle having its largestdimension smaller than or equal to about 100 nm.

As used herein, “semiconductor” refers to a material that has a band gapenergy that overlaps with the spectrum of solar radiation at the Earth'ssurface. In general, a semiconductor has a band gap between that of ametal and that of an insulator, although it is appreciated that there isno rigorous distinction between insulators and wide-gap semiconductors.

As used herein, “defect” or “dopant” refers to a crystallographicdefect, where the arrangement of atoms or molecules in a crystallinematerial departs from perfection by addition or exclusion of an ion,impurity atom, or small clusters of ions or atoms. The defect can occurat a single lattice point in the form of a vacancy, an interstitialdefect, or an impurity. In some embodiments, the crystalline lattice hassmall clusters of atoms that form a separate phase (i.e., aprecipitate).

The defect can be associated with a semiconductor nanocrystal. When thedefect is an atom or a cluster of atoms, “associated with” refers to anatom or a cluster of atoms that is incorporated into the semiconductornanocrystal, adsorbed onto, or otherwise associated (e.g., ionicallybound, covalently bound) to the surface of the semiconductornanocrystal. When the defect is a lattice vacancy, “associated with”refers to a lattice vacancy that is present within or on a surface ofthe semiconductor nanocrystal.

As used herein, “cluster of atoms” refers to an aggregate of atoms, theaggregate has a maximum dimension of less than or equal to 1 nm.

As used herein, light of shorter wavelength is considered “blue,”“bluer,” or “blue-shifted” when compared to light of a longerwavelength, which is “red,” “redder,” or “red-shifted,” even if thespecific wavelengths compared are not technically blue or red.

As used herein, “heterostructure” refers to a particle including atleast two crystalline materials having an interface between the twocrystalline materials.

As used herein, “alloy” refers to a single-phase mixture or asingle-phase solid solution including at least two different materials(e.g., a semiconductor material).

As used herein, “passivation” refers to coating of a protective materialor layer of molecules that reduces or eliminates deleterious surfacetrap states and protects against corrosion events such as oxidation.

As used herein, “core-shell” refers to an onion-like structure where ananocrystal or nanoparticle has a central core surrounded by one or moreconcentric shell layers.

As used herein, “aliovalent” substitution is where the atom that isreplacing an original atom in a crystal lattice has a differentoxidation state as the atom it is replacing.

As used herein, “isovalent” substitution is where the atom that issubstituting the original atom in a crystal lattice has the sameoxidation state as the atom it is replacing.

As used herein, “capping molecule” or “surface-capping molecule” refersto a molecule on a surface of a nanoparticle having a functional groupthat is covalently or non-covalently bound (e.g., ionically bound, boundvia hydrogen-bonds or van der Waals interactions) to the nanoparticlevia the functional group.

As used herein, “average maximum dimension” refers the average maximumlength of a nanoparticle or nanocrystal, obtained by measuring a maximumdimension (along any given direction) of each nanoparticle ornanocrystal in an ensemble of nanoparticles or nanocrystals, andaveraged amongst the measured nanoparticles or nanocrystals. Thedimension can be measured by various techniques including transmissionelectron microscopy or scanning electron microscopy, and the ensemble ofnanoparticles or nanocrystals used for this determination typicallyincludes at least 100 nano crystals.

As used herein, “surface roughness” refers the average root-mean-squareddeviation in the height of a surface over an area of approximately 100microns².

As used herein, “optical quantum efficiency” (OQE) refers to thefraction of incident photons absorbed by the photoluminescent species(e.g., nanoparticles) in an LSC that is emitted from the concentratoredge.

As used herein, “photoluminescence quantum yield” or “quantum yield”(QY) refer to the ratio of the number of emitted photons per number ofabsorbed photons.

As used herein, “oligomer” or “polymer” refers to a molecule havingbetween 3 and 10,000 constitutional units.

As used herein, the term “copolymer” refers to a polymer that is theresult of polymerization of two or more different monomers. The numberand the nature of each constitutional unit can be separately controlledin a copolymer. The constitutional units can be disposed in a purelyrandom, an alternating random, a regular alternating, a regular block,or a random block configuration unless expressly stated to be otherwise.A purely random configuration can, for example, be:x-x-y-z-x-y-y-z-y-z-z-z . . . or y-z-x-y-z-y-z-x-x . . . . Analternating random configuration can be: x-y-x-z-y-x-y-z-y-x-z . . . ,and a regular alternating configuration can be: x-y-z-x-y-z-x-y-z . . .. A regular block configuration has the following general configuration:. . . x-x-x-y-y-y-z-z-z-x-x-x . . . , while a random block configurationhas the general configuration: . . .x-x-x-z-z-x-x-y-y-y-y-z-z-z-x-x-z-z-z- . . . .

As used herein, the term “constitutional unit” of an oligomer or polymerrefers an atom or group of atoms in an oligomer or polymer, including apart of the chain together with its pendant atoms or groups of atoms, ifany. The constitutional unit can refer to a repeat unit. Theconstitutional unit can also refer to an end group on an oligomer orpolymer chain. For example, the constitutional unit of polyethyleneglycol can be —CH₂CH₂O— corresponding to a repeat unit, or —CH₂CH₂OHcorresponding to an end group.

As used herein, the term “repeat unit” corresponds to the smallestconstitutional unit, the repetition of which constitutes a regularmacromolecule (or oligomer molecule or block)

As used herein, the term “end group” refers to a constitutional unitwith only one attachment to an oligomer or polymer chain, located at theend of an oligomer or polymer. For example, the end group can be derivedfrom a monomer unit at the end of the oligomer or polymer, once themonomer unit has been polymerized. As another example, the end group canbe a part of an initiating agent that was used to synthesize thepolymer.

As used herein, the term “substituted” or “substitution” is meant torefer to the replacing of a hydrogen atom with a substituent other thanH. For example, an “N-substituted piperidin-4-yl” refers to replacementof the H atom from the NH of the piperidinyl with a non-hydrogensubstituent such as, for example, alkyl.

As used herein, the term “alkyl” refers to a straight or branched chainfully saturated (no double or triple bonds) hydrocarbon (carbon andhydrogen only) group. Examples of alkyl groups include, but are notlimited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl,sec-butyl, tertiary butyl, pentyl and hexyl. As used herein, “alkyl”includes “alkylene” groups, which refer to straight or branched fullysaturated hydrocarbon groups having two rather than one open valencesfor bonding to other groups. Examples of alkylene groups include, butare not limited to methylene, —CH₂—, ethylene, —CH₂CH₂—, propylene,—CH₂CH₂CH₂—, n-butylene, —CH₂CH₂CH₂CH₂—, sec-butylene, and—CH₂CH₂CH(CH₃)—. An alkyl group of this disclosure may optionally besubstituted with one or more fluorine groups.

As used herein, “alkenyl” refers to an alkyl group having one or moredouble carbon-carbon bonds. Examples of alkenyl groups include ethenyl,propenyl, and the like. The term “alkenylenyl” refers to a divalentlinking alkenyl group.

As used herein, “haloalkyl” refers to an alkyl group having one or morehalogen substituents. Example of haloalkyl groups include CF₃, C₂F₅,CHF₂, CCl₃, CHCl₂, C₂Cl₅, and the like.

As used herein, the term “aryl” refers to monocyclic or polycyclic(e.g., having 2, 3 or 4 fused rings) aromatic hydrocarbons such as, forexample, phenyl, naphthyl, anthracenyl, phenanthrenyl, indanyl, andindenyl. In some embodiments, aryl groups have from 6 to about 20 carbonatoms.

As used herein, the term “halo” or “halogen” includes fluoro, chloro,bromo, and iodo.

As used herein, “heteroaryl” groups refer to an aromatic heterocyclehaving at least one heteroatom ring member such as sulfur, oxygen, ornitrogen. Heteroaryl groups include monocyclic and polycyclic (e.g.,having 2, 3 or 4 fused rings) systems. Examples of heteroaryl groupsinclude without limitation, pyridyl, pyrimidinyl, pyrazinyl,pyridazinyl, triazinyl, furyl, quinolyl, isoquinolyl, thienyl,imidazolyl, thiazolyl, indolyl, pyrryl, oxazolyl, benzofuryl,benzothienyl, benzthiazolyl, isoxazolyl, pyrazolyl, triazolyl,tetrazolyl, indazolyl, 1,2,4-thiadiazolyl, isothiazolyl, benzothienyl,purinyl, carbazolyl, benzimidazolyl, indolinyl, and the like. In someembodiments, the heteroaryl group has from 1 to about 20 carbon atoms,and in further embodiments from about 3 to about 20 carbon atoms. Insome embodiments, the heteroaryl group contains 3 to about 14, 3 toabout 7, or 5 to 6 ring-forming atoms. In some embodiments, theheteroaryl group has 1 to about 4, 1 to about 3, or 1 to 2 heteroatoms.

As used herein, “heterocycloalkyl” refers to non-aromatic heterocyclesincluding cyclized alkyl, alkenyl, and alkynyl groups where one or moreof the ring-forming carbon atoms are replaced by a heteroatom such as anoxygen, nitrogen, or sulfur atom. Heterocycloalkyl groups can be mono-or polycyclic (e.g., having 2, 3, 4 or more fused rings or having a2-ring, 3-ring, 4-ring spiro system (e.g., having 8 to 20 ring-formingatoms). Heterocycloalkyl groups include monocyclic and polycyclicgroups. Example “heterocycloalkyl” groups include morpholino,thiomorpholino, piperazinyl, tetrahydrofuranyl, tetrahydrothienyl,2,3-dihydrobenzofuryl, 1,3-benzodioxole, benzo-1,4-dioxane, piperidinyl,pyrrolidinyl, isoxazolidinyl, isothiazolidinyl, pyrazolidinyl,oxazolidinyl, thiazolidinyl, imidazolidinyl, and the like. Ring-formingcarbon atoms and heteroatoms of a heterocycloalkyl group can beoptionally substituted by oxo or sulfido. Also included in thedefinition of heterocycloalkyl are moieties that have one or morearomatic rings fused (i.e., having a bond in common with) to thenonaromatic heterocyclic ring, for example phthalimidyl, naphthalimidyl,and benzo derivatives of heterocycles such as indolene and isoindolenegroups. In some embodiments, the heterocycloalkyl group has from 1 toabout 20 carbon atoms, and in further embodiments from about 3 to about20 carbon atoms. In some embodiments, the heterocycloalkyl groupcontains 3 to about 14, 3 to about 7, or 5 to 6 ring-forming atoms. Insome embodiments, the heterocycloalkyl group has 1 to about 4, 1 toabout 3, or 1 to 2 heteroatoms. In some embodiments, theheterocycloalkyl group contains 0 to 3 double bonds. In someembodiments, the heterocycloalkyl group contains 0 to 2 triple bonds.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art. Although methods and materials similar or equivalent to thosedescribed herein can be used in the practice or testing of the presentdisclosure, suitable methods and materials are described below. Allpublications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety. Incase of conflict, the present specification, including definitions, willcontrol. In addition, the materials, methods, and examples areillustrative only and not intended to be limiting.

Photoluminescent Nanoparticles

In one aspect, the present disclosure describes an LSC includingphotoluminescent nanoparticles that are suspended in a waveguidematerial or applied to a surface of a waveguide material. Thephotoluminescent nanoparticles include a semiconductor nanocrystal andone or more defects. The defect and the semiconductor nanocrystaltogether produce a photoluminescent effect. The defect is an atom, acluster of atoms, a lattice vacancy, or a combination thereof. As anexample, a given semiconductor nanocrystal can include one type ofdefect, such as an atom, a cluster of atoms, or a lattice vacancy. Asanother example, a given semiconductor nanocrystal can include two ormore types of defects.

In some embodiments, the defect is an atom.

In some embodiments, the defect is a cluster of atoms.

In some embodiments, the defect is a lattice vacancy.

In some embodiments, the defect includes an atom and a cluster of atoms.

In some embodiments, the defect includes an atom and a lattice vacancy.

In some embodiments, the defect includes a cluster of atoms and alattice vacancy.

In some embodiments, the defect includes an atom, a cluster of atoms,and a lattice vacancy.

The defect can be associated in any manner with the semiconductornanocrystal. When the defect is an atom or a cluster of atoms, thedefect can be incorporated into the semiconductor nanocrystal, adsorbedonto, or otherwise associated (e.g., ionically bound, covalently bound)to the surface of the semiconductor nanocrystal. When the defect is alattice vacancy, the defect can be present within or on a surface of thesemiconductor nano crystal.

An exemplary LSC 50 is shown in FIG. 2. LSC 50 includes a waveguidelayer 53 having a plurality of photoluminescent nanoparticles 54 in awaveguide material 52. In certain embodiments, the photoluminescentnanoparticles 54 are “guests” in a “host” waveguide material 52 (i.e., a“guest-host” system), as illustrated in FIG. 2. In other embodiments,the entire waveguide layer 53 is formed from a single material that hasphotoluminescent nanoparticles 54 attached (e.g., conjugated) to thewaveguide material 52.

The photoluminescent nanoparticles 54 absorb incident light 56 andphotoluminesce emitted light 58. The incident light 56 has a wavelengthλ₁ and emitted light 58 has a wavelength λ₂ that is longer than λ₁.

As used herein, with regard to planar waveguides, the planar surface(such as surface 60) is sometimes referred to as a “major surface” ofthe waveguide or device. A typical planar waveguide has two majorsurfaces (e.g., a top and bottom surface). A planar waveguide has minorsurfaces at the edges. In a typical LSC based on a planar waveguide,light from the luminophores is collected by PVs in optical communicationwith the edge(s) of the LSC planar waveguide.

In the exemplary LSC illustrated in FIG. 2, the incident light 56 entersthe waveguide layer 53 through the major surface 60. After conversion ofthe light via phosphorescence or fluorescence, the emitted light 58 isessentially trapped in the waveguide layer 53 by total internalreflection. A portion of the emitted light 58 travels through thewaveguide layer 53 to an edge 62 of the waveguide layer 53, where it isutilized by a light conversion device 64.

The photoluminescent nanoparticles are characterized by very largeenergy shifts between absorption and photoluminescence maxima arisingfrom rapid nonradiative energy localization at the defect. In someembodiments, the photoluminescent nanoparticles can luminesce followingthe light absorption and emission energy diagram provided in FIG. 3A.Referring to FIG. 3A, absorption of a photon hv by a semiconductornanocrystal can produce an exciton whose energy is rapidly andefficiently transferred to an excited state of the defect. Followingenergy transfer, the excited defect can then radiatively relax to aground state. The defect can have an absorption cross-section atemission wavelengths that is more than approximately 10³ times smaller(e.g., approximately 10⁴ times smaller, approximately 10⁵ times smaller,or approximately 10⁶ times smaller) than that of the associatedsemiconductor nanocrystal, while also having a high efficiencyluminescence from an excited state whose energy lies below the bandgapof the semiconductor nanocrystal. In some embodiments, the defect'sexcited state can relax radiatively in a near-complete or completemanner, with small to negligible nonradiative decay at ambienttemperatures. Referring again to FIG. 3A, the emissive excited state canbe sufficiently lower in energy compared to the lowest excited state ofthe semiconductor nanocrystal such that thermally-activated transfer ofenergy back to the nanocrystal is negligible at temperatures nearambient temperature. This can help achieve negligiblethermally-activated nonradiative decay, enabling very high QYs.

As an example, referring to FIG. 3B, time-resolved photoluminescencedecay curves of an exemplary photoluminescent nanoparticle(Mn:Zn_(1-x)Cd_(x)Se/ZnS) are shown at 290K and 77K. Negligiblethermally-activated nonradiative decay can be seen by the nearlyidentical decay curves at 290K and 77K. The nanoparticles have a 100%photoluminescence QY.

The photoluminescent nanoparticles can be monolithic nanocrystals. Asused herein, the term “monolithic” refers to a crystal that consists ofa single type of material. For example, referring to FIG. 4A, aphotoluminescent nanoparticle 100 (e.g., Zn_(1-x-y)Cd_(x)Mn_(y)Se) ismonolithic and has one material 102. Monolithic photoluminescentnanoparticles include two or more elements, and can include dopants, aswell.

In other embodiments, the photoluminescent nanoparticles include a coreand one or more shell layers, as illustrated in FIGS. 4B and 4C. Anynumber of shell layers may be incorporated. Referring to FIGS. 4B and4C, the composition of the core 112 or 122 and the first shell layer 114or 124 (closest to the core), respectively is different. Subsequentshell layers can be the same material as the core or different. Asillustrated in the FIG. 4B, in certain embodiments, two shell layers 114and 118 have the same composition and are separated by at least oneshell layer 116 having a different composition. An example of acore-shell semiconductor nanocrystal includes ZnSe/ZnS. Another exampleof a core-shell semiconductor nanocrystal includes ZnSe/ZnS/CdS/ZnS,where each “I” denotes a heterostructure or a gradient alloy ofmaterials forming the semiconductor crystal and the first compoundindicates the core. For example, ZnSe/ZnS/CdS/ZnS represents ananoparticle having a ZnSe core that is overlaid with a first layer ofZnS, a second layer of CdS, and a third layer of ZnS.

The semiconductor nanocrystal of the photoluminescent nanoparticle,whether monolithic or having a core-shell structure, can include any ofa variety of semiconductor materials. For example, the semiconductornanocrystal can include semiconductor II-VI materials (e.g., CdSe, CdS,CdTe, ZnSe, ZnS, ZnTe, alloys thereof, and heterostructures thereof);semiconductor I-VI materials (e.g., InN, InP, AlGaAs, InGaAs, alloysthereof, and heterostructures thereof); semiconductor I-VI materials(e.g., CuS, Ag₂S, alloys thereof, and heterostructures thereof); and/orcompound semiconductor materials (e.g., CuInSe₂, CuInS₂ and In₂S₃,alloys thereof, and heterostructures thereof). In some embodiments, thephotoluminescent nanoparticle includes CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe,InN, InP, AlGaAs, InGaAs, CuS, Ag₂S, CuInSe₂, CuInS₂, In₂S₃, GaP, InP,GaN, AlN, GaAs, PbS, PbSc, PbTc, CuCl, Cu₂S, Cu₂Sc, Cu₂ZnSnS₄,Cu₂ZnSnSc₄, Cu₂ZnSnTe₄, CuInTe₂, Si, Ge, Y₂O₃, Y₂S₃, Y₂Se₃, NaYF₄,NaYS₂, LaF₃, YF₃, ZnO, TiO₂, La₂O₂S, Y₂O₂S, Gd₂O₂S, Zn₃N₂, Zn₃P₂, and/oralloys or heterostructures thereof. The semiconductor materials can beused singly or in any combination.

The photoluminescent nanoparticles have a small amount of defect that isin intimate contact with the semiconductor nanocrystal. The defect caninclude an atom, or a cluster of atoms, at any oxidation state. Forexample, the atom or cluster of atoms can include an emissivetransition-metal element (e.g., Mn, Co, Cr, V, Fe, Ni, and/or Cu), adonor-acceptor luminescence activator element (e.g., Ag, Au, Al, Bi, Sb,Cl, Br, and/or I), and/or certain lanthanide elements (e.g., Ce, Pr, Nd,Sm, Eu, Tb, Dy, Ho, Er, Tm, or Yb). In some embodiments, the atom orcluster of atoms can include Mn, Co, Cu, Pt, Ru, V, Cr, Ag, Au, Al, Bi,Sb, Cl, Br, I, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and/or Yb. Forexample, the atom or cluster of atoms can include Mn²⁺, Yb³⁺, Nd³⁺, Cu⁺,Cl⁻, Ag⁺, Er³⁺, Eu²⁺, and/or Cr³⁺.

In some embodiments, the defect is Mn.

In some embodiments, the defect is Cu.

In some embodiments, the defect is a lanthanide. It is believed thatlanthanides can have sharp emission spectra that can extend in somecases into the near-infrared. For example, Yb^(3|) undergoes efficientluminescence at 980 nm, making it well-matched to the peak responsivityof Si PVs.

The atom or cluster of atoms can be located at a regular lattice site ina crystal structure of a given semiconductor crystal (i.e., asubstitutional defect). In some embodiments, the atom or cluster ofatoms can be found at or close to the lattice site(s) of the atom(s) itis replacing. In some embodiments, the defect can be an interstitialdefect, where an atom occupies a site in the crystal structure at whichthere is usually not an atom. The substitution of atom or cluster ofatoms can be isovalent or aliovalent.

Without wishing to be bound by theory, it is believed that one or moremechanisms contribute to a reduction of self-absorption, when the defectis an atom or cluster of atoms. For example, an optically-inducedelectronic transition from a nanocrystal ground state to an excitedemissive state can be spin-forbidden in a photoluminescent nanoparticle(e.g., Mn-doped ZnSe (Mn:ZnSe)). As another example, a low concentrationof an emissive species can result in reduced self-absorption (e.g.,Cu-doped CdSe (Cu:CdSe)). As yet another example, a large excited-statenuclear reorganization of an emissive species can reduce self-absorption(e.g., Cu-doped CdSe (Cu:CdSe)). In some embodiments, first-order parityforbiddenness of an optically induced electronic transition from ananocrystal ground state to an excited emissive state can limitself-absorption (e.g., Yb-doped PbS (Yb:PbS)). Other mechanisms may alsocontribute to reduced self-absorption.

In some embodiments, the defect is a lattice vacancy. The latticevacancy can include an atomic vacancy, where an atom is missing from thecrystallographic lattice of the semiconductor crystal (i.e., a Schottkydefect). For example, in a CdSe crystal, the atomic vacancy can be anabsence of Cd and/or Se at their corresponding lattice sites. In someembodiments, the lattice vacancy is an oxygen vacancy, a seleniumvacancy, a sulfur vacancy, a phosphorus vacancy, or a tellurium vacancy.

In some embodiments, the photoluminescent nanoparticles are Mn-dopedZnSe/ZnS/CdS/ZnS (Mn:ZnSe/ZnS/CdS/ZnS), Cu-doped InP/ZnS (Cu:InP/ZnS),Zn_(1-x-y)Cd_(x)Mn_(y)Se/ZnS, Yb-doped Si/SiO₂ (Yb: Si/SiO₂), Yb-dopedNaYF₄/CdSe/ZnSe (Yb: NaYF₄/CdSe/ZnSe), Cu_(x)Zn_(y)In_(z)Se_(2-δ),and/or Yb-doped CdTe/ZnS (Yb:CdTe/ZnS). In some embodiments, when thephotoluminescent nanoparticles are Zn_(1-x-y)Cd_(x)Mn_(y)Se/ZnS, x and ycan have values such that 1−x−y>x+y and x>y. For example, x can be about0.1 and y can be about 0.01 in Zn_(1-x-y)Cd_(x)Mn_(y)Se/ZnS. In someembodiments, when the photoluminescent nanoparticles areCu_(x)Zn_(y)In_(z)Se_(2-δ), the ratio of y:z can range from about 0(e.g., from about 0.1, from about 0.2, from about 0.3, or from about0.4) to about 0.5 (e.g., to about 0.4, to about 0.3, to about 0.2, or toabout 0.1), and x/(y+z) can be about 0.05). As used herein, when aformula includes a semicolon “;”, it is understood that the elementspreceding the semicolon represent the dopant, while elements followingthe semicolon represent the semiconductor crystal. “/” denotes aheterostructure or a gradient alloy of materials forming thesemiconductor crystal.

In some embodiments, the photoluminescent nanoparticles are Mn-dopedZnSe. The Mn-doped ZnSe photoluminescent nanoparticles can optionallyinclude one or more layers (e.g., a passivation layer, a wide-gapnanocrystal layer, a capping molecule layer, and/or an inorganicmolecule layer) on the surface of the photoluminescent nanoparticles,such as a ZnS layer.

In some embodiments, the photoluminescent nanoparticles are Mn-dopedZnSe/ZnS.

In some embodiments, the photoluminescent nanoparticles are Mn-dopedCdZnSe. The Mn-doped ZnSe/ZnS photoluminescent nanoparticles canoptionally include one or more layers (e.g., a passivation layer, awide-gap nanocrystal layer, a capping molecule layer, and/or aninorganic molecule layer) on the surface of the photoluminescentnanoparticles, such as a ZnS layer.

In some embodiments, the photoluminescent nanoparticles are Mn-dopedCdZnSe/CdS.

In some embodiments, the photoluminescent nanoparticles are Cu-dopedCdSe. The Cu-doped CdSe photoluminescent nanoparticles can optionallyinclude one or more layers (e.g., a passivation layer, a wide-gapnanocrystal layer, a capping molecule layer, and/or an inorganicmolecule layer) on the surface of the photoluminescent nanoparticles,such as a CdS layer.

In some embodiments, the photoluminescent nanoparticles are Cu-dopedCdSe/CdS.

In some embodiments, the photoluminescent nanoparticles areZn_(1-x-y)Cd_(x)Mn_(y)Se, where x and y have values such that 1−x−y>x+yand x>y. For example, x can be about 0.1 and y can be about 0.01 inZn_(1-x-y)Cd_(x)Mn_(y)Se/ZnS. The Zn_(1-x-y)Cd_(x)Mn_(y)Sephotoluminescent nanoparticles can optionally include one or more layers(e.g., a passivation layer, a wide-gap nanocrystal layer, a cappingmolecule layer, and/or an inorganic molecule layer) on the surface ofthe photoluminescent nanoparticles.

In some embodiments, the defect is present at a mole fraction of 10% orless (e.g., 8% or less, 6% or less, 4% or less, 2% or less, or 1% orless) and/or 0.05% or more (e.g., 1% or more, 2% or more, 4% or more, 6%or more, or 8% or more) relative to the constituents of (i.e., the atomsforming) the semiconductor nanocrystal. For example, a cation defect canbe present in a cation mole fraction of about 1% in the semiconductornano crystal.

In some embodiments, the photoluminescent nanoparticle includes apassivation layer on a surface of the nanocrystal. The passivation layercan include a material such as CdSc, CdS, CdTc, ZnSc, ZnS, ZnTc, InN,InP, AlGaAs, InGaAs, CuS, Ag₂S, CuInSc₂, CuInS₂, In₂S₃, GaP, InP, GaN,AlN, GaAs, PbS, PbSe, PbTe, CuCl, Cu₂S, Cu₂Se, Cu₂ZnSnS₄, Cu₂ZnSnSe₄,Cu₂ZnSnTe₄, CuInTe₂, Si, Ge, Y₂O₃, Y₂S₃, Y₂Se₃, NaYF₄, NaYS₂, LaF₃, YF₃,ZnO, TiO₂, La₂O₂S, Y₂O₂S, Gd₂O₂S, Zn₃N₂, Zn₃P₂, and/or alloys thereof.For example, a CdS passivation layer can be present on a surface of aCdSe nanocrystal, a GaP passivation layer can be present on a surface ofan InP nanocrystal, an alloy of ZnSe and CdS can be present on a surfaceof a CdSe nanocrystal. A given surface layer can be a suitablepassivation layer if it is capable of isolating the photoluminescentnanoparticle from surface or external species that can directly orindirectly reduce the nanocrystal photoluminescence QY. Common criteriafor an effective nanoparticle passivation layer include a material thatis more robust against oxidation, that has a wider band gap than thenanocrystal, and/or that has no low-lying electronic states.

In some embodiments, the photoluminescent nanoparticle further includesa wide-gap nanocrystal, which has an energy gap that is larger than atargeted incident photon's energy. The wide-gap nanocrystal can form aheterostructure with a semiconductor of a different composition, toprovide absorption of a targeted incident photon. For example, thewide-gap nanocrystal can include NaYF₄, Gd₂O₂S, Y₂S₃, and/or CePO₄. Thewide-gap nanocrystal can include a dopant, such as Yb, which can bepresent at a mole fraction of 10% or less (e.g., 8% or less, 6% or less,4% or less, 2% or less, or 1% or less) and/or 0.05% or more (e.g., 1% ormore, 2% or more, 4% or more, 6% or more, or 8% or more) relative to thewide-gap semiconductor nanocrystal. For example, a cation dopant can bepresent in a cation mole fraction of about 1% relative to theconstituents of (i.e., the atoms forming) the wide-gap nanocrystal.

In some embodiments, the photoluminescent nanoparticle further includesa capping molecule on a surface (i.e., a surface-capping molecule). Aplurality of capping molecules can form a continuous or discontinuouslayer on the photoluminescent nanoparticle. For example, the cappingmolecule can be an amine, an ammonium, a carboxylate, a phosphonate, aphosphine, a phosphine oxide, an oligomeric phosphine, a silane, athiol, a dithiol, a disulfide, an N-containing heteroaryl, and/or anN-containing heterocycloalkyl, each of which is independently optionallysubstituted with a C₄₋₂₀ alkyl, C₄₋₂₀ alkenyl, aryl group, and anycombination thereof. In some embodiments, a substituted capping moleculeprovides increased solubility in a waveguide matrix. In someembodiments, the capping molecule is dodecylamine, trioctylamine,oleylamine, trioctylphosphonate, trioctylphosphine oxide,trioctylphosphine, pyridine, acetate, stearate, myristate, and/oroleate. In certain embodiments, the capping molecule includes a reactivefunctional group such as an olefin, a silane, an acrylate, and/or anepoxide, wherein the reactive functional group can form a covalent bondwith a polymer such as a polysilane, a polyacrylate polymer, apolyolefin, and/or a perfluorinated polyether.

The capping molecule on a photoluminescent nanoparticle surface can bean oligomer or a polymer. For example, the capping molecule can includean oligomer that includes functional groups located at the end groups ofthe oligomer, and/or as side chains on the repeat units of the oligomer.The functional groups can include phosphines, phosphine oxides,phosphonates, phosphine oxides, pyridines, amines, amides, carboxylicacids, and/or carboxylates such acetates, stearates, myristates, and/oroleates. Each of the phosphines, phosphine oxides, phosphonates,phosphine oxides, pyridines, amines, amides, carboxylates, andcarboxylic acids can be independently optionally substituted with C₄₋₂₀alkyl, C₄₋₂₀ alkenyl, aryl, or any combination thereof. An oligomer orpolymer on a surface of the photoluminescent nanocrystal can includepolysiloxanes, polyalkylphosphines, and/or polyarylphophines, which caninclude pendant groups such as alkyl phosphines, aryl phosphines, alkylphosphine oxides, aryl phosphine oxides, phosphonates, pyridines,amines, amides, carboxylic acids, carboxylates, and/or thiols.

In certain embodiments, one or more capping molecule can be replacedwith an inorganic molecule such as BF₄ ⁻, S²⁻, HS⁻, Se²⁻, HSe⁻, Te²⁻,HTe⁻, TeS₃ ²⁻, OH⁻, and NH₂ ⁻, In₂Se₄ ²⁻, SnTe₄ ⁴⁻, AsS₃ ³⁻, Sn₂S₆ ⁴⁻,SCN⁻, and/or related inorganic molecules thereof, on a surface of thephotoluminescent nanoparticle. In some embodiments, the relatedinorganic molecules include Sn₂Se₆ ⁴⁻, In₂S₄ ²⁻, Ge₂Se₄ ²⁻, SnSe₄ ²⁻,etc. It is believed that a wide variety of inorganic small-moleculecapping agents can be used for the replacing the capping molecule.Examples of inorganic small molecules suitable for replacing the cappingmolecule are described in Nag A., et al., Metal-free Inorganic Ligandsfor Colloidal Nanocrystals: S²⁻, HS⁻, Se²⁻, HSe⁻, Te²⁻, HTe⁻, TeS₃ ²⁻,OH⁻, and NH₂ ⁻as Surface Ligands, J. Am. Chem. Soc. 2011, 133, 10612.Kovalenko M. V. et al., Colloidal Nanocrystals with Molecular MetalChalcogenide Surface Ligands, Science 2009, 324, 1417-1420, Rosen E. L.et al., Exceptionally Mild Reactive Stripping of Native Ligands fromNanocrystal Surfaces by Using Meerwein's Salt, Angewandte Chemie,International Edition, 2012, 51, 684-689, each of which is hereinincorporated by reference in its entirety. The inorganic small moleculecapping agents can be used to provide solubility to the photoluminescentnanoparticle in polar solvents, and can render the photoluminescentnanoparticle more suitable for incorporating into oxides or otherinorganic waveguides.

The photoluminescent nanoparticles can have a maximum dimension of about100 nm or less, about 50 nm or less, about 10 nm or less, about 5 nm orless, or about 2 nm or less. In some embodiments, the photoluminescentnanoparticles have a maximum dimension of about 5 nm or less. In otherembodiments, the photoluminescent nanoparticles have an average maximumdimension ranging from about 1 nm to about 100 nm (e.g., from about 1 nmto about 75 nm, from about 1 nm to about 50 nm, from about 1 nm to about20 nm, from about 1 nm to about 10 nm, from about 1 nm to about 5 nm,from about 5 nm to about 75 nm, from about 5 nm to about 50 nm, fromabout 5 nm to about 25 nm, from about 5 nm to about 20, or from about 5nm to about 10 nm.) The photoluminescent nanoparticle dimensions can bedetermined, for example, by transmission electron microscopy or scanningelectron microscopy, or by x-ray diffraction line broadening (e.g., forsmall nanoparticles). Without wishing to be bound by theory, it isbelieved that light scattering processes can occur with increasingprobability as a photon's optical path through a given materialcontaining photoluminescent nanoparticles increases, when the index ofrefraction of the photoluminescent nanoparticle is different than thatof the material in which the photon travels. A photoluminescentnanoparticle having a smaller maximum dimension can reduce lightscattering. As used herein when describing a dimension, measurement,etc., the term “about” indicates a possible difference of +/−5%.

When the photoluminescent nanoparticle is a core/shell composition, thecore can have a maximum dimension of about 100 nm or less, about 50 nmor less, about 10 nm or less, about 5 nm or less, or about 2 nm or less.In some embodiments, the core has a maximum dimension of about 5 nm orless. In other embodiments, the core has an average maximum dimension offrom about 1 nm to about 100 nm. In some embodiments, each shell canhave an average thickness of from about 0.2 nm to about 25 nm (e.g.,from about 0.2 nm to about 15 nm, from about 0.2 nm to about 10 nm, fromabout 5 nm to about 25 nm, from about 5 nm to about 15 nm, from about0.5 nm to about 15 nm, from about 10 nm to about 15 nm). Shellthicknesses can be determined by measuring the size of the particlesbefore and after shell growth, for example, using electron microscopy.

In some embodiments, the combined core and shell dimension is such thatthe total maximum nanoparticle dimension is sufficiently small to havedecreased (e.g., avoid) photon scattering. For example, for dopedsemiconductor nanoparticles in polymer matrices, the combined core andshell dimension can be 10 nm or less.

Semiconductor materials and dopants are also described in U.S. PatentApplication Publication No. 2013/0140506, filed May 14, 2012, theentirety of which is incorporated herein by reference.

Optical Characteristics and Advantages

Without wishing to be bound by theory, it is believed that asemiconductor nanocrystal's bandgap can be tuned by varying the size ofthe nanocrystal, and/or by replacing certain elements of a semiconductornanocrystal by alloying and/or or making a core-shell structure. Forexample, a Mn:ZnSe photoluminescent nanoparticle can have an emissiveexcited state produced by energy transfer from the ZnSe nanocrystalwhose bandgap is size dependent. Other semiconductors materials can havedifferent bandgaps. For example, by replacing some or all of the Zn withCd in ZnSe nanocrystals by alloying (forming Zn_(1-x)Cd_(x)Senanocrystals) or core/shell growth, the energy gap of the nanocrystalcan be narrowed to absorb substantially more of the solar spectrum (FIG.5), while still allowing essentially zero overlap between thesemiconductor nanocrystal host absorption and Mn²⁺ luminescence. In someembodiments, the photoluminescent nanoparticles can remain smallrelative to the wavelength of light (FIG. 6), can besolution-processable, and can be compatible with polymer or glass matrixformation.

In certain embodiments, there is little or no overlap between theemission and absorption spectra of a given photoluminescentnanoparticle. For example, the emission spectrum and lowest absorptionband of a given photoluminescent nanoparticle do not overlap by greaterthan 25% (e.g., greater than 20%, greater than 15%, greater than 10%,greater than 5%, greater than 2%) of their integrated normalized areas.In other embodiments, the absorption wavelength range and the emissionwavelength range of the photoluminescent nanoparticle do not overlap atany wavelength where the absorption extinction coefficient exceeds 1000M⁻¹cm⁻¹ (in nanocrystal molarity).

In some embodiments, the absorption coefficient of a photoluminescentnanoparticle at an energy equal to the nanoparticle bandgap energyexceeds the absorption coefficient at an energy equal to the maximalemission by a factor of at least 500 (e.g., at least 750, at least 1000,at least 1500, or at least 2000), such that the photoluminescentnanoparticle is a good absorber for incident photons but does not havesignificant absorbance at the emission wavelengths.

The photoluminescent nanoparticles can reduce photon losses fromself-absorption and scattering compared to other LSC luminophores,including other types of photoluminescent nanoparticles. As will bediscussed in Example 2, infra, when compared to known core-shellphotoluminescent nanoparticles, the photoluminescent nanoparticles ofthe present disclosure demonstrate smaller waveguide losses.

A material (e.g., an LSC) that includes the photoluminescentnanoparticle can have an optical transmittance of 90% or more (95% ormore, or 97% or more) below the nanoparticle bandgap energy. Dependingon an application for the photoluminescent nanoparticles, differentcombinations of transparency ranges and emission ranges can apply. Forexample, if the photoluminescent nanoparticles are to be used in a fullytransparent window, the material that includes the nanoparticles canhave a 90% or greater optical transmittance between 400 nm and 800 nm.If the photoluminescent nanoparticles are to be used in a partiallytransparent window, the material that includes the nanoparticles canhave a 10% or greater (e.g., 25% or greater, 50% or greater, or 75% orgreater) optical transmittance between 400 nm and 800 nm. If thephotoluminescent nanoparticles are to be used for non-windowapplications, the material that includes the nanoparticles can exhibitless than 10% optical transmittance at energies higher than the bandgap.

In some embodiments, the material that includes the photoluminescentnanoparticles has an optical transmittance of 10% or less at energiesgreater than the nanoparticle bandgap energy. Such a material can beused for applications where it is more preferable to maximally absorbsolar irradiance than to provide partial transparency at energiesgreater than the bandgap. For example, an LSC applied to a rooftop canprovide greater benefit from maximally absorbing solar irradiance thanby providing partial transparency. In another example, an LSC havingtransmittance of 10% or less at energies greater than the nanoparticlebandgap energy can be used to filter UV solar photons, and/or to harvestthe maximum amount of solar energy.

The photoluminescent nanoparticles can have a QY of greater than 50%(e.g., greater than 75%, or equal to 100%).

The photoluminescent nanoparticles of the present disclosure arewell-suited for incorporation into a waveguide matrix for LSCapplications. For example, the photoluminescent nanoparticles luminescewith high QYs and little to no self-absorption, which allows very largehigh-efficiency LSCs to be made. This can be important to reducing thecost of solar electricity generated using an LSC.

Another advantage of the photoluminescent nanoparticles of the presentdisclosure is that the range of wavelengths (color) of light absorbed bythe photoluminescent nanoparticles can be tuned from the ultraviolet tothe near-infrared by controlling their size, structure, and chemicalcomposition. This enables favorable matching to the solar spectrum,thereby increasing or optimizing the fraction of sunlight that can beharvested. As an example, an LSC incorporating the photoluminescentnanoparticles that selectively absorb only UV or infrared light would beoptically transparent, but could still be used to generate electricity.As another example, using two or more different types ofphotoluminescent nanoparticles in a single LSC, it is possible to make adevice that achieves color-balanced absorption, such that the device canhave any desired color, including uniform grey. Hence, LSCs based onphotoluminescent nanoparticles would be suitable for use as a windowcoating or a versatile architectural material for building exteriors,roofing, etc. The minimal absorption and maximal emission energies ofseveral representative examples of the photoluminescent nanoparticles ofthe present disclosure are shown in FIG. 7, which demonstrates that awide range of absorption and emission energies can be obtained.Referring to FIG. 7, Cu⁺-doped CdSe or InP nanocrystals showdonor/acceptor recombination luminescence that is red-shifted from thesemiconductor absorption edge. Yb³⁺-doped PbS shows sharp sensitizedYb³⁺ f-f luminescence. These and related doped nanocrystals can improveLSC performance by harvesting a broader portion of the solar spectrumand providing better luminescence matching with Si photovoltaics, whileretaining the small reabsorption losses displayed by Mn²⁺-doped ZnSenanocrystals.

In some embodiments, several LSC layers, each containing a differentphotoluminescent nanoparticle and interfaced to a photovoltaic cell (PV)with a bandgap matched to the nanoparticle's emission can be stacked toincrease the overall solar energy conversion efficiency, analogously toa multi junction PV.

Yet another advantage of the photoluminescent nanoparticles is that thenanoparticles are more photochemically stable than most organicluminophores. This is important for producing a device that can functionoutdoors for many years, as required for most applications.

Yet a further advantage of the photoluminescent nanoparticles is thatthe nanoparticles can be processable in a number of solvents and by anumber of methods, allowing facile integration into plastic or glasswaveguide matrices at high photoluminescent nanoparticle concentration,and allowing co-deposition with the matrix material by scalablesolution-based methods including spray coating, ultrasonic spraycoating, spin coating, dip coating, infusion, roll-to-roll processing,and ink jet printing.

Incorporation of Photoluminescent Nanoparticles into Waveguide Materials

The photoluminescent nanoparticles can be incorporated into waveguidematerials for LSC applications. For example, the LSC can include aphotoluminescent nanoparticle to nanocrystal to waveguide materialweight ratio of less than 10% (e.g., less than 8%, less than 6%, lessthan 4%, less than 2%, or less than 1%) and/or more than 0.01% (morethan 1%, more than 4%, more than 6%, or more than 8%).

The waveguide matrix can be a polyacrylate, a polycarbonate, a cyclicperfluorinated polyether, a polysilicone, a polysiloxane, apolyalkylacrylate, a cyclic olefin (e.g., Zeonex COP™ (Nippon Zeon) andTopas COC™ (Celanese AG)), crosslinked derivatives thereof, and/orcopolymers thereof, wherein each of which is independently optionallysubstituted with a C₁₋₁₈ alkyl, C₁₋₁₈ alkenyl, aryl group, and anycombination thereof. In some embodiments, the waveguide material ispoly(methyl methacrylate) or poly(lauryl methacrylate), and cross-linkedderivatives thereof. In some embodiments, the waveguide material ispoly(lauryl methacrylate-co-ethylene glycol dimethacrylate). Thewaveguide matrix can be transparent.

In some embodiments, the waveguide material can be an inorganic glass, apolycrystalline solid, or an amorphous solid. Representative examples ofinorganic glass, polycrystalline solid, or amorphous solid includeindium tin oxide, SiO₂, ZrO₂, HfO₂, ZnO, TiO₂, fluorosilicates,borosilicates, phosphosilicates, fluorozirconates (e.g., ZBLAN(ZrF₄—BaF₂—LaF₃—AlF₃—NaF)), organically modified silicates (e.g.,silica-polyethylene urethane composites, silica-polymethylmethacrylatecomposites, and glymo-3-glycidoxypropyltrimethoxysilane). For example,the waveguide material can be an inorganic glass, such as indium tinoxide.

The waveguide material can have a surface roughness of 2 nm or less(e.g., 1 nm or less, 0.5 nm or less, or 0.2 nm or less). Without wishingto be bound by theory, it is believed that a waveguide material having asmooth surface can minimize light scattering events.

When the photoluminescent nanoparticles are incorporated into awaveguide material, the resulting LSC can have an OQE of up to about 75%at geometric gains exceeding 20. For example, the OQE of the LSC can begreater than 25% (e.g., greater than 50%, greater than 70%).

The effects of optical losses in an LSC resulting from absorption by thewaveguide material, scattering from waveguide imperfections, roughnessat the surface of the waveguide, and similar non-idealities can besummarized through a waveguide attenuation coefficient, α. Differentwaveguide materials can possess different attenuation coefficients, andto maximize OQE, α should be as small as possible. Referring to FIG. 8,the relative OQE of a square LSC measuring 1 m on each side is shown onthe y-axis for a representative group of waveguide materials, and thex-axis shows ranges of α that are generally applicable for eachmaterial. In some embodiments, the LSC having a plurality ofphotoluminescent nanoparticles can have an attenuation coefficient ofless than 0.05 dB/cm (less than 0.03 dB/cm, less than 0.01 dB/cm) at awavelength corresponding to a peak emission of the plurality ofphotoluminescent nanoparticles.

In some embodiments, in addition to minimizing waveguide losses due toabsorption by the waveguide material, scattering from waveguideimperfections, roughness at the surface of the waveguide, and similarnon-idealities, there may be other considerations in selecting awaveguide material, such as cost or environmental lifetime. Thecompatibility of the photoluminescent nanoparticles with a wide range ofwaveguide materials as described above can enable selection of awaveguide material that is well-suited for a particular LSC applicationof the LSC. As an example, referring again to FIG. 8, a large LSC canbenefit from a more highly transparent waveguide material such as apolysiloxane, whereas a less transparent (but also less expensive)waveguide material such as poly(methyl methacrylate) can be used for asmall LSC.

Architecture of LSCS

The LSC can be a single layer or a multilayer device. Configurations ofwaveguide materials in an LSC are described, for example, in U.S. PatentApplication Publication No. 2011/0253198, filed Mar. 4, 2011,incorporated herein by reference in its entirety.

Referring now to FIG. 9, a multilayer LSC device 150 is illustratedwherein a number of waveguide layers 155, 160, 165, and 170 are arrangedin a stack. Light of broad spectrum wavelengths λ_(L) is impinged on thedevice, and the luminophore layers 155, 160, 165, and 170 absorb andluminesce to direct light into the PV 175 to produce electrical current.The impinging light λ_(L) in the illustrated embodiment has at leastlight of wavelength λ_(L1), but may be a broader light source (e.g.,white light source) containing any number of wavelengths of light.However, for simplicity, the device 150 illustrated in FIG. 9 primarilydescribes the device with regard to input of light of wavelength λ_(L1).

The layer 155, 160, 165, and 170 of the device 150 are arranged from“bluest” (155) to “reddest” (170), although it will be appreciated theseterms are not indicative of the actual color of absorption or emissionof the layer, but only indicating that the “bluest” is the shortestwavelength of light (e.g., emitted light), and the “reddest” is thelight having the longest emission wavelength. Layers 155, 160, 165, and170 each include one or more luminophores of any type, provided that atleast one of layers 155, 160, 165, and 170 includes a photoluminescentnanoparticle.

The layers of the device 150 in FIG. 9 begin at the layer closest to thesource of light, which is layer 155, the bluest layer. The luminophores(e.g., the photoluminescent nanoparticles) in layer 155 have anabsorption at wavelength λ_(a1) and a luminescent emission at wavelengthλ_(e1).

Layer 160 of the device 150 is “bluer” in relation to layers 165 and170. The luminophores (e.g., the photoluminescent nanoparticles) of thelayer 160 have an absorption at wavelength λ_(a2) and a luminescentemission at λ_(e2).

Layer 165 of the device 150 is a layer wherein the luminophores are“redder” than the blue layers 155 and 160. The luminophores (e.g., thephotoluminescent nanoparticles) of layer 165 have an absorption atλ_(a3) and a luminescent emission at λ_(e3).

Layer 170 of the device 150 includes luminophores (e.g., thephotoluminescent nanoparticles) having the reddest emission of any ofthe layers of any of the device 150.

The device 150 harvests the energy of the incoming light λ_(L) accordingto at least the following exemplary description of operation. Initially,light of wavelength λ_(L1) impinges on the bluest isotropic layer 155.The absorption λ_(a1) of the bluest layer 155 overlaps with thewavelength of the incident light λ_(L1), thus causing the luminophore toluminesce and emit light of wavelength λ_(e1). The light at λ_(e1) isemitted in all directions, including some light that reaches the PV 175,some light that will emit back toward the light source, and some lightthat will emit in the direction of the bluer layer 160.

Light of wavelength λ_(e1) that crosses from the bluest layer 155 to thebluer layer 160 impinges on the luminophores in the bluer layer 160. Theabsorption wavelength λ_(a2) of the luminophores of the bluer layer 160overlap λ_(e1), causing luminescence at λ_(e2), once again in alldirections.

Light of wavelength λ_(e2) that crosses from the bluer layer 160 intothe redder layer 165 is absorbed at wavelength λ_(a3), which overlapswith λ_(e2). The luminophores in the redder layer 165 luminesce atwavelength λ_(e3) in all directions.

Light emitted from the redder layer 165 at wavelength λ_(e3) thatcrosses into the reddest layer 170 will be absorbed by the luminophoresin layer 170 at wavelength λ_(a4), which overlaps λ_(e3). Theluminophores then luminesce at wavelength λ_(e4).

In some embodiments, the luminophores in the reddest layer 170 areoriented according to their emission and absorption dipoles, asdescribed in U.S. Patent Application Publication No. 2011/0253198, filedMar. 4, 2011, herein incorporated by reference in its entirety. In suchan embodiment, the light at wavelength λ_(e4) is preferentially emittedin the plane of the layer, and therefore travels either toward or awayfrom the PV 175. Notably, light emitted at wavelength λ_(e4) does notemit in a direction either toward layer 165 or away from the device(toward the “bottom” of the device illustrated in FIG. 9). Accordingly,the oriented reddest layer 170 can acts as a barrier for containinglight within the device 150 at the bottom edge.

If the source of light λ_(L) is a broader wavelength source (e.g., whitelight), it will be appreciated that the luminophores in layers 160, 165,and 170 need not only harvest light directly from the layers precedingthem in the energy cascade, but can also absorb and luminesce based onlight directly impinging on the luminophores at the absorptionwavelength. This effect is illustrated in FIG. 9 with the dashed arrowsin layers 160, 165, and 170, wherein light of wavelength λ_(L2) canoptionally impinge on the luminophores in the bluer layer 160; light ofwavelength λ_(L3) can optionally impinge on the redder luminophores inlayer 165, and light of wavelength λ_(L4) can impinge on the reddestluminophores in reddest layer 170.

In some embodiments, instead of having a waveguide layer including aluminophore in layer 170, layer 170 can instead be a cladding layerhaving a refractive index that is less than a refractive index of layer160. Thus, the LSC can have one or more cladding layers in opticalcommunication with an outer layer of waveguide materials of the LSC. Thecladding layer can have a refractive index less than a refractive indexof the waveguide materials, such that the cladding layer causes light tobe confined within the waveguide materials by total internal reflection.

In some embodiments, instead of having a single PV abutting waveguidelayers 155, 160, 165, and 170, one or more of layers 155, 160, 165, and170 can each be in optical communication with a separate PV, which canhave a bandgap that matches the peak emission of the layer it is coupledto.

While FIG. 9 describes LSC devices having multiple layers ofluminophores wherein each layer includes a single luminophore, it willbe appreciated that a single waveguide layer can include multipleluminophores.

Referring now to FIG. 10, a “packaged” LSC device 200 is illustratedwherein a number of isotropic layers 210, 211, and 215 form a core 216including luminescent layers. The core 216 is bounded on the upper andlower ends by reddest layers 205 and 206. The bottom layer of the deviceis a mirror 219 configured to reflect at least the reddest wavelength.In a preferred embodiment, the mirror 219 reflects all wavelengths thatmay potentially impinge on the mirror 219.

In an alternative embodiment, mirror 219 can be either a mirror or ascattering surface.

In an exemplary device design, the mirror 219 is preferably separatedfrom layer 206 by either an air gap or a low-index layer (notillustrated). Direct contact with layer 206 would likely diminish thetotal internal reflection efficiency of the waveguiding in layer 206.

The top layer of the device is an encapsulation layer 217, which canserve to protect the device 200 from environmental conditions, such asmoisture, oxygen, and other damaging effects. The encapsulation layer217 may also be an anti-reflection layer that is configured to allowmaximum light impinging on the device 200 into the luminophorescontained within. The luminophore-containing layers 205, 206, and 216all abut a PV 220 for converting light emitted into electrical current.

In the device 200 illustrated in FIG. 10, the bluest layer 215 is in themiddle of the device, followed by, moving outward, redder layers 210 and211, and finally, bounded by reddest layers 205 and 206. It will beappreciated that the similarly named, but differently numberedcomponents of the device 200 can be either the same or different. Thatis, the redder layers 210 and 211 can be the same or different, as canthe reddest layers 205 and 206, etc.

As discussed above with regard to FIG. 9, when the reddest layer 170includes an oriented luminophore, layer 170 in the device 150 can act tocreate a boundary for the light contained within the device 150 so as tomaximize containment of the energy harvested by the device 150 andturned into electrical current by the PV 175. In some embodiments, thisconcept is extended in the device 200 illustrated in FIG. 10, whereinreddest layers 205 and 206 bound both the top and bottom of the device200 can include oriented luminophores, so as to create confinement atboth upper and lower surfaces of the device 200, thereby maximizing theamount of light energy retained within the device.

In certain embodiments, the luminophore layer with the longest emissionwavelength (e.g., the “reddest”) includes oriented luminophores.

In some embodiments, instead of having a single PV abutting waveguidelayers 205, 206, 210, 211, and 215, one or more of layers 205, 206, 210,211, and 215 can each be in optical communication with a separate PV,which can have a bandgap that matches the peak emission of the layer itis coupled to.

In some embodiments, instead of encapsulation layer 217 and mirror 219,one or both of layers 217 and 219 can instead be a cladding layer havinga refractive index that is less than a refractive index of an adjacentlayer (i.e., layer 205 and/or 206).

Fabrication of multilayer LSCs is accomplished using methods known tothose of skill in the art and disclosed herein. For example, layersincluding photoluminescent nanoparticles can be fabricated using knownmethods (e.g., spin coating, drop coating, evaporation, vapordeposition, and the like). Layers including oriented luminophore layerscan be fabricated using liquid crystals, extrusion, or other methodsdisclosed in U.S. Patent Application Publication No. 2011/0253198, filedMar. 4, 2011, herein incorporated by reference in its entirety.

A substrate may be used to support a layer (or layers) that are notself-supporting. A multilayer LSC can also or alternatively befabricated in two or more portions and joined (e.g., bonded) to form afinished device. The “stack” of luminophore-containing waveguides isedge-coupled to a PV cell to form a completed multilayer LSC system forproducing electricity.

While PVs are described above, it is understood that the LSC canadditionally or alternatively be in optical communication with one ormore other light-utilization devices such as a solar heater, aconcentrated solar thermal power system, a lighting device, and/or aphotochemical reactor.

Uses of the LSCs

Without wishing to be bound by theory, it is believed that theoreticalconcentration factors exceeding 100× are possible for an LSC of thepresent disclosure because the maximum optical gain is limited by thephotoluminescent nanoparticle's Stokes shift, rather than opticalfocusing.

The LSC can be made in a variety of colors, and can even be opticallytransparent, rendering the LSC useful as solar windows or otherbuilding-integrated architectural elements. The LSC can be configuredfor a variety of potential applications ranging from consumerelectronics to utility-scale solar farm deployment. Because thematerials and installation costs of an LSC are lower than those forconventional photovoltaic panels, solar electricity generated using anLSC has the potential to be significantly cheaper than other forms ofsolar power.

As an example, the LSC can be used for window applications. As discussedabove, the optical transmittance of a photoluminescent nanoparticle (andof the resulting LCS) can be tuned depending on the application. The LSCincluding photoluminescent nanoparticles can have a peak emission atwavelengths longer than 850 nm, so as to decrease visible “glow.” Insome embodiments, for window and other applications, visible glow isalternatively or additionally be reduced (or eliminated) byincorporating into a cladding layer an absorbing non-emissive speciessuch as an organic dye, whose absorption range can overlap with theemission range of the photoluminescent nanocrystals. The dye-containingcladding layer can be separated from the waveguide layer by a lowrefractive index layer, thereby absorbing light leaving the waveguidelayer out of its escape cone.

In some embodiments, for optimal interfacing of the LSC to siliconphotovoltaics, a maximal emission wavelength is between 900-1100 nm. Ingeneral, optimal interfacing to a photovoltaic device is achieved whenthe emission maximum of the photoluminescent nanocrystal is slightlylower in energy than the bandgap of the photovoltaic.

In some embodiments, the LSC form or is part of a coating for a device.

In some embodiments, the LCS form or is part of a free-standing polymerfilm.

In some embodiments, the LCS form part of an electronic display or atouch screen.

The following examples are included for the purpose of illustrating, notlimiting, the described embodiments.

EXAMPLES Example 1. Preparation and Characterization of RepresentativePhotoluminescent Nanoparticles

In this example, the preparation, characterization, and use ofrepresentative photoluminescent nanoparticles of the disclosure aredescribed.

Fabrication and Characterization of PhotoluminescentNanoparticle-Containing LSCs

LSCs were prepared and characterized based on several representativeexamples of photoluminescent nanoparticles with different structures andcompositions. Mn:ZnSe/ZnS core-shell photoluminescent nanoparticles weregrown by thermal decomposition of the tetramer (Me₄N)₂[Zn₄(SePh)₁₀] inhexadecylamine at 270° C. under a pressure of N₂ gas for 30 to 60minutes while also in the presence of MnCl₂. The resulting nanocrystalswere then coated with a ZnS shell by successive slow additions of zincoleate and trioctylphosphine sulfide in a solution of octadecene andoleylamine at 230° C. under a pressure of N₂ gas for 5 to 25 minutes.Particles were suspended in toluene and repeatedly washed using ethanoland methanol.

Mn:ZnSe/ZnS/CdS/ZnS multishell photoluminescent nanoparticles wereprepared in a similar way except after the addition of the first ZnSshell there were successive additions of a CdS and ZnS shells. Theseshells were deposited by successive slow additions of cadmium oleate andtrioctylphosphine sulfide or zinc oleate and trioctylphosphine sulfide.Particles were suspended in toluene and repeatedly washed using ethanoland methanol. Zn_(1-x-y)Cd_(x)Mn_(y)Se (x and y have values such that1−x−y>x+y and x>y. Here, x is approximately 0.1 and y is approximately0.01) photoluminescent nanoparticles were grown by thermal decompositionof the tetramer (Me₄N)₂[Zn₄(SePh)₁₀] in hexadecylamine at 270° C. undera pressure of N₂ gas for 30 to 60 minutes while also in the presence ofboth CdCl₂ and MnCl₂. Particles were suspended in toluene and repeatedlywashed using ethanol and methanol.

Cu-doped CdSe photoluminescent nanoparticles were prepared by dissolvingcadmium acetate and oleic acid in hexadecane and degassing under N₂. Tothis mixture, copper stearate was added against positive nitrogenpressure and the solution was degassed again. After heating to 180° C.,selenium powder in TOP was rapidly injected into the cadmium solution,the nanoparticles were allowed to grow, and the reaction was thencooled. The nanoparticles were purified by washing with ethanol:acetone,centrifugation, and redispersion in toluene. In another synthesis, SeO₂,cadmium myristate, and CuCl in were added to octadecene, degassed, andheated to 250° C., where oleic acid was added dropwise and the reactionwas cooled following nanoparticle formation. The nanoparticles werepurified by washing with ethanol:acetone, centrifugation, andredispersion in toluene

Using these photoluminescent nanoparticles two groups of LSC deviceswere prepared, one based on photoluminescent nanoparticle-containingpolymer films, the other employing photoluminescentnanoparticle-containing liquid filled planar cells. Results from thesedevices will now be described.

Polymer Film LSCs

Polymer devices were prepared by dispersing a toluene solution ofphotoluminescent nanoparticles into laurylmethacrylate containing ˜1 wt% of Irgacure 651 photoinitiator. Several droplets of the resultingmixture were placed on a clean borosilicate glass coverslip, which wasthen covered by a second coverslip and the resultingglass/solution/glass samples were photopolymerized to form a solid film.Samples measured 2.5×7.5×0.16 cm in size, resulting in a geometric gain,defined as the facial-to-edge area ratio, G=20 when the entire LSC isilluminated. LSCs were uniformly illuminated with 400 nm monochromaticlight and emission from an aperture of length l=0.7 cm centered in oneedge was collected by an integrating sphere, passed through amonochrometer, and detected by a photomultiplier tube. The remainingdevice edges were blackened.

The optical quantum efficiency (OQE), defined as the fraction ofabsorbed photons reaching a device edge, was determined. To compute it,two corrections to the measured edge emission intensity were required toaccount for: (1) collection of light from only a portion of the wholeperimeter, and (2) detection of only a certain fraction of photonsreaching the device edge using the experimental arrangement (somephotons travel in directions lying outside the edge escape cone andhence upon reaching an edge are reflected back into the waveguide). Inactual application, the LSC would usually have one or moreperimeter-mounted PVs whose refractive index exceeded that of the LSCmatrix (e.g. Si or GaInP); under such conditions all photons reachingthe edge could in principle be captured. Applying these corrections theefficiency was computed using the procedure described in Erickson, C. S.et al., Zero-Reabsorption Doped-Nanocrystal Luminescent SolarConcentrators, ACS Nano 2014, 8, 3461, incorporated herein in itsentirety.

In some measurements, successively larger areas of the device wereilluminated using a movable mask, enabling determination of OQE as afunction of geometric gain. FIG. 11 shows the experimental arrangementused to perform these measurements.

Representative absorption and emission spectra from three polymer filmdevices prepared from three types of photoluminescent nanoparticles areshown in FIGS. 12A-12C. In each case absorption was dominated by thesemiconductor nanocrystal (the sensitizer) and emission was dominated bythe dopant (the activator, either Mn^(2|) or Cu^(|)). In one case(Cu:CdSe/CdS, FIG. 12C) some excitonic emission can be observed. Thisresults in some overlap with the absorption spectrum, producing someself-absorption in this case. In the other two cases (i.e., Mn:ZnSe/ZnSand Mn:CdZnSe/CdS, FIGS. 12A and 12B, respectively) shown there isessentially zero reabsorption. The optical quantum efficiencies forthese devices are listed in Table 1. Some other similarly prepareddevices (data not shown) exhibited optical quantum efficiencies up to68% at a geometric gain of 10, among the highest ever reported, andclose to the theoretical maximum of ˜75%, corresponding to ahypothetical device with directionally isotropic luminophore emission,unity photoluminescence QY, and no self-absorption. This exceptionallyhigh level of performance could be attributed to the high QY of theincorporated photoluminescent nanoparticles used to prepare it, whichwas measured to be 97%. This illustrated the importance of QY on LSCperformance, which should be as high as possible. Devices prepared fromphotoluminescent nanoparticles having lower QY (for example, aliquid-filled LSC device based on Mn:ZnSe/ZnS, which had a measured QYof the incorporated photoluminescent nanoparticles of 50%) performedless efficiently than comparable devices based on photoluminescentnanoparticles having higher QY.

TABLE 1 Device Attenuation coefficient LSC based on configuration OQE atG = 20 (dB/cm) Mn: ZnSe/ZnS Polymer film 54% 0.009 Mn: CdZnSe/ZnSPolymer film 35% 0.004 Cu: CdSe/CdS Polymer film 4.1%  0.39

FIGS. 13A-13C show the OQE variation with geometric gain for thesedevices. Referring to FIGS. 13A-13C, OQE is observed to graduallydecrease as the geometric gain is increased. From the rate of decreasethe attenuation coefficient, α, was computed following a proceduredescribed in Erickson C. S. et al., Zero-Reabsorption Doped-NanocrystalLuminescent Solar Concentrators”, ACS Nano 2014, 8, 3461., incorporatedherein in its entirety, with the results summarized in Table 1.

FIGS. 14A-14C show emission spectra collected through the edge apertureas a function of geometric gain for these LSCs. The spectra have beennormalized to exhibit a peak value of 1, enabling comparison of the peakposition and shape. It is seen that for LSCs incorporating Mn:CdZnSe/ZnS(FIG. 14A) and Mn:ZnSe/ZnS (FIG. 14B) neither the peak position norshape change with increasing geometric gain. The spectra were nearlyindistinguishable, i.e., unchanged as a function of distance. Thisbehavior was in marked contrast to that normally observed in an LSCwhere self-absorption leads to a progressive red-shifting ofedge-emitted light as a function of excitation distance from the edge.The normalized emission spectra for an LSC incorporating Cu:CdSe/CdSphotoluminescent nanoparticles shown in FIG. 14C displayed two emissionfeatures: an emission feature centered at approximately 580 nm was dueto band edge emission, and a second broader and more intense emissionfeature centered at approximately 700 nm was due to emission from the Cudopant. Because the band edge feature partially overlaps withCu:CdSe/CdS absorption the emission becomes progressively moreattenuated with increasing geometric gain. The majority of emittedphotons, however, were associated with emission from the dopant. Theseunderwent essentially no self-absorption as evidenced by a lack of anysignificant change in the emission band shape or position withincreasing geometric gain.

Liquid Filled LSCs

Liquid-filled LSCs were constructed from two rectangular glasscoverslips separated by a spacer and filled with solutions ofphotoluminescent nanoparticles dispersed in toluene. Cells wererectangular in shape, measuring 2.5 cm×7.5 cm×0.065 cm and the thicknessof the photoluminescent nanoparticle-containing liquid layer was 0.033cm. After filling, cells were sealed around their perimeters usingoptical adhesive and the full perimeter except for the segment used tomeasure edge emission was blackened.

Edge emission was measured by illuminating LSCs with a point excitationsource positioned a distance d from the collection edge. Light leavingthe collection edge was captured in an integrating sphere and passedthrough a monochrometer before being detected by a photomultiplier tube.OQE was computed as

${{OQE} = \frac{{agN}_{em}}{N_{abs}}},$where N_(em) is the number of photons collected from the edge aperture,

$N_{abs} = \frac{P\;\lambda}{{hc}\left( {1 - 10^{- A}} \right)}$is the number of photons absorbed by the sample (P and λ are the powerand wavelength of the excitation source, A is the LSC absorbance at theexcitation wavelength measured by ultraviolet-visible spectroscopy, h isPlanck's constant and c is the speed of light). The constant a is theinverse of the fraction of photons reaching the perimeter that arrivetraveling within the edge escape cone. Its value was computed viaballistic Monte Carlo simulations employing a modified version of themodel of McDowall S. et al., Appl. Opt. 2013, 52, 1230-1239, hereinincorporated by reference in its entirety, yielding a=1/0.43=2.33. Theconstant g^(g) corrected for the collection of light from a smallsegment l^(l) of the perimeter. It was computed using the approximation

$g = {\frac{2\sqrt{\pi\; A}}{l} =}$to account for point illumination. Devices were fabricated using twotypes of photoluminescent nanoparticles: (a) Mn-doped core-shellZnSe/ZnS and (b) alloyed Zn_(1-x-y)Cd_(x)Mn_(y)Se.

TABLE 2 Device LSC based on configuration OQE Mn: ZnSe/ZnS Liquid filled51% Zn_(1−x−y)Cd_(x)Mn_(y)Se Liquid filled 22%

Representative absorption and emission spectra from two devices areshown in FIGS. 15A (Mn:ZnSc/ZnS) and 15B (Zn_(1-x-y)Cd_(x)Mn_(y)Sc). Asin the case of polymer-based devices, absorption was dominated by thesemiconductor nanocrystal sensitizer and emission is dominated by the Mnactivator. Average optical quantum efficiencies for the two devices werelisted in Table 2.

To illustrate the effects of the near total lack of self-absorption bythe luminophore, FIGS. 16A (Zn_(1-x-y)Cd_(x)Mn_(y)Se) and 16B(Mn:ZnSe/ZnS) show normalized emission spectra collected from the deviceedge for five different excitation distances ranging from about 5 mm toabout 45 mm from the point of collection in steps of about 8 mm. Foreach device, the spectra were nearly indistinguishable, i.e., unchangedas a function of distance. As noted above, this behavior was in markedcontrast to that normally observed in an LSC where self-absorption leadto a progressive red-shifting of edge-emitted light as a function ofexcitation distance from the edge.

Further illustrating the performance benefits of photoluminescentnanoparticle-based LSCs, FIG. 17 shows the OQE for the same two devices,determined as a function of excitation distance. The efficiency remainedessentially unchanged with distance, again in marked contrast to thebehavior observed in LSCs based on other luminophore types where the OQEdecreased approximately exponentially with excitation distance from theedge. This indicated that very large area devices could be made withoutsignificant degradation of performance.

Example 2: Comparison of Photoluminescent Nanoparticles of the PresentDisclosure with Known Photoluminescent Nanoparticles

This Example evaluates whether the photoluminescent nanoparticles of thepresent disclosure can reduce losses from self-absorption and scatteringcompared to other LSC luminophores, including other types ofphotoluminescent nanoparticles.

Referring to FIG. 18, the normalized waveguided photoluminescenceintensities measured in a linear, rod-shaped LSC as a function of thedistance between excitation and collection positions for two LSCsincorporating different types of photoluminescent nanoparticles wereevaluated. The first LSC included Mn:Zn_(1-x)Cd_(x)Se/ZnS (where x isapproximately 0.1) photoluminescent nanoparticles, synthesized asdescribed in Example 1. The second LSC included CdSe/CdS nanoparticles,which are examples of a “giant” shell core-shell semiconductornanocrystal, a type of photoluminescent nanoparticle known to possesssmall self-absorption, prepared as described in Chen, O. et al., Nat.Mater. 2013, 12, 445-451. Both nanoparticle waveguides have an opticaldensity of 1 over a 1 mm path length at the nanocrystal bandgap energy.The data show considerably smaller waveguide losses for the disclosedphotoluminescent nanoparticles.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

The invention claimed is:
 1. A luminescent solar concentrator,comprising: (a) a plurality of photoluminescent nanoparticles, eachcomprising: (i) a semiconductor nanocrystal; and (ii) a nanocrystaldefect, wherein the nanocrystal defect and the semiconductor nanocrystalcombine to produce a photoluminescence effect and wherein the defect isselected from the group consisting of an atom, a cluster of atoms, alattice vacancy, and any combination thereof; and (b) a waveguidematerial having the plurality of photoluminescent nanoparticlessuspended therein or applied to a surface of the waveguide material,wherein when exposed to sunlight, the semiconductor nanocrystal of thephotoluminescent nanoparticles absorbs energy in the form of lighthaving a first wavelength, the absorbed energy is transferred to thenanocrystal defect, the nanocrystal defect then spontaneously emitslight having a second wavelength longer than the first wavelength intothe waveguide material, the light having the second wavelength iscaptured by total internal reflection in the waveguide material, and thelight having the second wavelength then travels through the waveguidematerial and is emitted by the waveguide material or opticallycommunicated to a light-utilization device.
 2. The luminescent solarconcentrator of claim 1, wherein the nanocrystal defect is locatedwithin or on a surface of the semiconductor nanocrystal.
 3. Theluminescent solar concentrator of claim 1, wherein the photoluminescentnanoparticle comprises a material selected from the group consisting ofCdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, InN, InP, AlGaAs, InGaAs, CuS, Ag₂S,CuInSe₂, CuInS₂, In₂S₃, GaP, InP, GaN, AN, GaAs, PbS, PbSe, PbTe, CuCl,Cu₂S, Cu₂Se, Cu₂ZnSnS₄, Cu₂ZnSnSe₄, Cu₂ZnSnTe₄, CuInTe₂, Si, Ge, Y₂O₃,Y₂S₃, Y₂Se₃, NaYF₄, NaYS₂, LaF₃, YF₃, ZnO, TiO₂, La₂O₂S, Y₂O₂S, Gd₂O₂S,Zn₃N₂, Zn₃P₂, alloys thereof, heterostructures thereof, and anycombination thereof.
 4. The luminescent solar concentrator of claim 1,wherein each photoluminescent nanoparticle further comprises apassivation layer on a surface.
 5. The luminescent solar concentrator ofclaim 4, wherein the passivation layer comprises a passivation materialselected from CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, InN, InP, AlGaAs,InGaAs, CuS, Ag₂S, CuInSe₂, CuInS₂, In₂S₃, GaP, InP, GaN, AN, GaAs, PbS,PbSe, PbTe, CuCl, Cu₂S, Cu₂Se, Cu₂ZnSnS₄, Cu₂ZnSnSe₄, Cu₂ZnSnTe₄,CuInTe₂, Si, Ge, Y₂O₃, Y₂S₃, Y₂Se₃, NaYF₄, NaYS₂, LaF₃, YF₃, ZnO, TiO₂,La₂O₂S, Y₂O₂S, Gd₂O₂S, Zn₃N₂, Zn₃P₂, alloys thereof, and any combinationthereof.
 6. The luminescent solar concentrator of claim 1, wherein eachphotoluminescent nanoparticle further comprises a capping molecule on asurface.
 7. The luminescent solar concentrator of claim 6, wherein thecapping molecule is selected from the group consisting of an amine, acarboxylate, a phosphonate, a phosphine, a phosphine oxide, anoligomeric phosphine, a thiol, a dithiol, a disulfide, an N-containingheterocycle, and any combination thereof.
 8. The luminescent solarconcentrator of claim 6, wherein the capping molecule is selected fromthe group consisting of dodecylamine, trioctylamine, oleylamine,trioctylphosphonate, trioctylphosphine oxide, trioctylphosphine,pyridine, acetate, stearate, myristate, and oleate.
 9. The luminescentsolar concentrator of claim 6, wherein the capping molecules comprise areactive functional group selected from the group consisting of olefin,silane, acrylate, or epoxide, and any combination thereof.
 10. Theluminescent solar concentrator of claim 1, wherein each photoluminescentnanoparticle further comprises an inorganic molecule selected from thegroup consisting of BF₄ ⁻, S²⁻, HS⁻, Se²⁻, HSe⁻, Te²⁻, HTe⁻, TeS₃ ²⁻,OH⁻, and NH₂ ⁻, In₂Se₄ ²⁻, SnTe₄ ⁴⁻, AsS₃ ³⁻, Sn₂S₆ ⁴⁻, SCN⁻, andrelated inorganic molecules thereof, on a surface of thephotoluminescent nanoparticle.
 11. The luminescent solar concentrator ofclaim 1, wherein each photoluminescent nanoparticle further comprises anpolymeric molecule that comprises a side chain comprising a moietyselected from the group consisting of phosphines, phosphine oxides,phosphonates, phosphine oxides, pyridines, amines, amides, carboxylicacids, carboxylates, thiols, and any combination thereof, on a surfaceof the photoluminescent nanoparticle.
 12. The luminescent solarconcentrator of claim 1, wherein each photoluminescent nanoparticlecomprises a core-shell structure.
 13. The luminescent solar concentratorof claim 1, wherein the atom or cluster of atoms is selected from thegroup consisting of Mn, Co, Cu, Pt, Ru, V, Cr, Ag, Au, Al, Bi, Sb, Cl,Br, or I, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb.
 14. Theluminescent solar concentrator of claim 1, wherein the atom or clusterof atoms comprises Mn.
 15. The luminescent solar concentrator of claim1, wherein the lattice vacancy is an atomic vacancy.
 16. The luminescentsolar concentrator of claim 1, wherein the defect is present at a molefraction of 10% or less.
 17. The luminescent solar concentrator of claim1, wherein the defect is present in a mole fraction of about 1%.
 18. Theluminescent solar concentrator of claim 1, wherein the defect is one ofan atom, a cluster of atoms, or a lattice vacancy.
 19. The luminescentsolar concentrator of claim 1, wherein the defect is two or more of anatom, a cluster of atoms, and a lattice vacancy.
 20. The luminescentsolar concentrator of claim 1, wherein each photoluminescentnanoparticle further comprises a wide-gap nanocrystal selected from thegroup consisting of NaYF₄, Gd₂O₂S, NaYS₂, Y₂S₃, and CePO₄.
 21. Theluminescent solar concentrator of claim 20, wherein the wide-gapnanocrystal comprises a dopant.
 22. The luminescent solar concentratorof claim 21, wherein the dopant comprises Yb.
 23. The luminescent solarconcentrator of claim 1, wherein the photoluminescent nanoparticles areselected from the group consisting of Mn-doped ZnSe/ZnS/CdS/ZnS,Cu-doped InP/ZnS, Zn_(1-x-y)Cd_(x)Mn_(y)Se/ZnS, Yb-doped Si/SiO₂,Yb-doped NaYF₄/CdSe/ZnSe, Cu_(x)Zn_(y)In_(z)Se_(2-δ), and Yb-dopedCdTe/ZnS.
 24. The luminescent solar concentrator of claim 1, wherein thephotoluminescent nanoparticles have an average maximum dimension of 10nm or less.
 25. The luminescent solar concentrator of claim 1, whereinthe photoluminescent nanoparticles have an optical transmittance of 90%or more below the nanoparticle bandgap energy.
 26. The luminescent solarconcentrator of claim 1, wherein the photoluminescent nanoparticles havean optical transmittance of 10% or less above the nanoparticle bandgapenergy.
 27. The luminescent solar concentrator of claim 1, wherein anabsorption coefficient of the photoluminescent nanoparticles at anenergy equal to the nanoparticle bandgap energy exceeds an absorptioncoefficient at an energy equal to the maximal emission by a factor of atleast
 500. 28. The luminescent solar concentrator of claim 1, whereinthe photoluminescent nanoparticles have an absorption spectrum and anemission spectrum, and the emission spectrum and lowest absorption banddo not overlap by greater than 25% in their integrated normalized areas.29. The luminescent solar concentrator of claim 1, wherein the waveguidematerial is selected from the group consisting of a polyacrylate, apolycarbonate, an inorganic glass, a polycrystalline solid, an amorphoussolid, a cyclic perfluorinated polyether, a polysilicone, apolysiloxane, a polyalkylacrylate, a cyclic olefin.
 30. The luminescentsolar concentrator of claim 1, wherein the waveguide material ispoly(methyl methacrylate), poly(lauryl methacrylate), or cross-linkedderivatives thereof.
 31. The luminescent solar concentrator of claim 1,wherein the waveguide material has a surface roughness of 2 nm or less.32. The luminescent solar concentrator of claim 1, comprising aphotoluminescent nanoparticle to waveguide material weight ratio of lessthan 10%.
 33. The luminescent solar concentrator of claim 1, wherein theluminescent solar concentrator has an optical quantum efficiency of upto about 75% at geometric gains exceeding
 20. 34. The luminescent solarconcentrator of claim 1, having an attenuation coefficient of less than0.05 dB/cm at a wavelength corresponding to a peak emission of theplurality of photoluminescent nanoparticles.
 35. The luminescent solarconcentrator of claim 1, further comprising at least one cladding layerin optical communication with the waveguide material, wherein thecladding layer has a refractive index less than a refractive index ofthe waveguide.
 36. The luminescent solar concentrator of claim 1,further comprising a light-utilization device in optical communicationwith the waveguide material, wherein the light-utilization device isselected from the group consisting of a photovoltaic cell, a solarheater, a concentrated solar thermal power system, a lighting device,and a photochemical reactor.
 37. A window pane comprising theluminescent solar concentrator of claim
 1. 38. A coating comprising theluminescent solar concentrator of claim
 1. 39. A free-standing polymerfilm comprising the luminescent solar concentrator of claim
 1. 40. Anelectronic display comprising the luminescent solar concentrator ofclaim
 1. 41. A touch screen comprising the luminescent solarconcentrator of claim
 1. 42. The luminescent solar concentrator of claim1, wherein waveguide material is planar.
 43. The luminescent solarconcentrator of claim 1, wherein the waveguide has two major surfacesand one or more minor surfaces.
 44. The luminescent solar concentratorof claim 1, wherein the photoluminescent particles comprise CuInS₂,CuInSe₂, alloys thereof, or heterostructures thereof; and have a defectselected from an aliovalent Cu atom, a lattice vacancy, and acombination thereof.